It is reported that materials used for solvent resistant membranes are cross-linked organic polymers, ceramics or inorganic compounds. However, these materials are often expensive and are limited to ultrafiltration or microfiltration, not available in the nanofiltration range. Polymeric materials used for nanofiltration membranes generally swell in organic solvents. The swelling of such membranes in the presence of solvents and under pressure usually results in compaction and loss of flux and performance. Due to hydrophobicity of these materials, most of organic solvents may result in significant loss of flux in an application of polyamide nanofiltration composite membranes for the treatment of water. The conventional nanofiltration composite membranes, therefore, are incongruent in the areas of food technology, the treatment of waste streams, chemical process, and petrochemical industry. Hydrophobic nanofiltration composite membrane having organic solvent resistance which is called ‘Membrane D’ produced by Membrane Osmonics Co. has been commercialized but has poor rejection rate due to flexibility of silicone.
There are references relating to organic solvent resistant separation membranes as follows. U.S. Pat. No. 5,032,282 discloses that composite membranes are organic solvent resistant but they have high permeation of aqueous solution due to hydrophilic surface property of the membranes, thus resulting in poor permeation of organic solvents. U.S. Pat. No. 6,113,794 discloses that composite nanofiltration membranes coated with chitosan which is hydrophilic have excellent resistant in various organic solvents. However, fluxes of other solvents were much lower than that of water and especially hexane flux was not observed. Therefore, the composite membrane disclosed in U.S. Pat. No. 6,113,794 is not suitable for hydrophobic solvents.
On the other hand, the nanofiltration composite membrane of the present invention is stable in various solvents, which has been proved by investigating the solvent permeation behavior through the membrane, and provides high permeation of hydrophobic solvents and good rejection rate of an organic compound dissolved in an organic solvent.